It has been known for some time that the management of solid particles, for example, sands, asphaltenes, clays, drill cuttings, and scale particles, discharged from hydrocarbon producing wells, has an important impact on downstream processing equipment. Some typical problems associated with downstream processing are:                Wear on valves, particularly choke pressure control valves        Drop out of solids in processing vessels such as three phase separators        Wear on pumps and rotating equipment        The management of separated solids on the seabed following sub-sea processing        Disposal of solids, which are contaminated with oil offshore.        
With the advent of directional drilling, which is used to seek out and produce ever more recoverable hydrocarbons from old or marginal reservoirs, and the use of lateral completions, ever increasing volumes of solids are being produced.
The use of a well head de-sander in the form of a solid/liquid cyclone housed in a pressure vessel is well known and has been utilised successfully in the offshore oil and gas exploration and production industry. A well head de-sander is typically designed to cope with slowly increasing flow rate, in order to match the field production profile of a reservoir's life. Therefore, the design must have a sufficient turndown (ie, minimum to maximum flow capacity whilst remaining an efficient separator).
Up to now, this is often been achieved in one of two ways, either by taking the de-sander off-line and changing the number of cyclone liners before returning it on-line, or by directing the flow to one or more cyclone vessels as required.
In the first method, a cyclone vessel may have the ability to hold, for example, forty cyclone liners, but has only twenty liners installed with twenty blank liners. When the flow rate through the cyclone vessel increases to the point where the pressure drop across the cyclone vessel is too high, the operator takes the cyclone vessel off line, opens it and installs more cyclone liners.
In the second method, the same effect can be achieved by having two cyclone vessels, for example, each with twenty liners installed on a skid system with valves installed on the skid manifold to allow selection of the vessels and hence number of cyclone liners on line at a given time.
In a typical example system, a well produces a volume of liquids with some associated gas, for example, 50 m3/hr. The pressure available for use by the cyclone de-sander is typically 1 bar. Cyclone liner design characteristics determine that the efficient maximum flow rate per liner whilst meeting its d90 cut size is given by the available pressure drop allowed divided by a constant depending on the cyclone shape size and efficiency. In this example a cyclone liner is chosen that has a maximum flow rate under these conditions of 10 m3/hr whilst separating 90% of all particles 20 microns and above that have a density equal to or greater than 2000 kg/m3. The field's initial flow rate is considered to be 30 m3/hr and after one years operation is likely to increase to 50 m3/hr.
The cyclone vessel is therefore initially filled with three cyclone liners and two blanks. Once the pressure drop through the cyclone vessel increases to above 1 bar, the vessel is taken offline, the two blanks are removed and two extra cyclone liners are installed. The cyclone vessel is then brought back on line.
When considering a gas field, the problem of flow and pressure change is exaggerated because the gas production often begins as a low volume dense phase fluid at high pressure, and as the field matures the volume increases and the pressure drops. This therefore requires either a smaller cyclone liner type on start up, which will need to be replaced later in the field's life with a larger cyclone, or a high number of extra smaller cyclone liners in separate vessels or added to a single vessel as needs be.
Whereas these systems have had some success to date onshore and on topsides offshore, these systems are not practicable when considering sub-sea processing, because the retrieval of a cyclone vessel for maintenance and/or the use of divers in deep sea areas are not viable.
It is therefore an object of the invention to provide a new cyclone assembly, which has an improved turndown, ie, minimum to maximum flow capacity whilst remaining an efficient separator.